An Antitail is a term used in astronomy to describe one of the two tails which visibly display from a comet as it passes close to the Sun. Streams of dust and gas emanate from a comet and each form their own distinct tails, pointing in different directions.
A tail of visible fine dust with particles no bigger than that in smoke is blown from the comet and often forms a noticeable curved tail called the antitail. This tail is fainter and its particles, are accelerated by the suns rays away from the comet nucleus at an oblique angle to the sun's rays.
At the same time, the brighter ion tail, made of gases, always points directly away from the Sun, as the gas is more strongly affected by the solar wind than is dust, and follows magnetic field lines rather than an orbital trajectory. This is the tail most often associated with a comet in imagery.
Coarse particles, the size of pebbles and sand, continue to follow the comet's orbit and these form the dust trail. This in usually invisible from Earth except in infrared light.
The antitail is normally visible for a brief interval only when the Earth passes through the comet's orbital plane. 
- The coma and tail at the main Comet article.